Power switches are used in electronic systems for selective distribution of electrical power. One of the simplest forms of power switch is the MOS transistor. When used as a power switch, one of the source/drain terminals of an MOS transistor forms the input terminal of the power switch, and the other one of the source/drain terminals of the MOS transistor forms the output terminal of the power switch. When the MOS transistor is turned on by a voltage applied between its gate and source terminals, a low resistance path forms between its drain and source terminals, thereby connecting the input and output terminals of the switch together.
FIG. 1 is a schematic diagram of a switching circuit 100, as known in the prior art. When the ON terminal 115 is pulled to a logic high level, under-voltage detector 103, over-voltage detector 105, and over-temperature detector 107 are powered up to check the input voltage level and the temperature. The decisions of these detectors appear at their respective output terminals as logic signals. If all of the checked conditions are determined to be within allowable limits, AND gate 109 turns on power switch 101, thereby enabling load 113 to be connected to input power source 111.
FIG. 2 is a transistor schematic diagram of power switch 101 according to one implementation. As shown, power switch 101 (hereinafter alternatively referred to as switch) includes a PMOS transistor 201 that performs the switching operation, and a driver circuitry that includes PMOS transistor 203 and NMOS transistor 205. Transistor 205 is adapted to pull the gate terminal of transistor 201 to the ground potential when a high voltage level is applied to its EN terminal, thus turning transistor 201 on. Transistor 203 is adapted to pull the gate terminal of transistor 201 to the potential of the IN terminal when a low voltage level is applied to its EN terminal, which in turn, turns off transistor 201. Switching circuit 100 does not have current limiting capability.
FIG. 3 is a transistor schematic diagram of switch 101 (see FIG. 1) that has current limiting capability, according to another implementation. Switch 101 of FIG. 3 includes a sense resistor 403, an amplifier 405, and transistors 201, 409. Sense resistor RSNS 403 senses current ISW flowing through transistor 201. The voltage drop across resistor 403 is thus proportional to the current flowing through transistor 201. Voltage source 407 represents a threshold voltage VTH establishing the maximum current IMAX allowed to flow through transistor 201. Amplifier 405's output signal OA is low when (ISW*R403)<VTH, where R403 represents the resistance of resistor 403. Switch 101 is on when transistor 201 is turned on by the low level of signal OA generated by amplifier 405.
When an output overload condition demands a switch current higher than IMAX, amplifier 405 increases the gate voltage of transistor 201 to keep the switch current regulated at IMAX which is defined by:IMAXPA=VTH/RSNS  (3)
When enabling/disabling signal EN is asserted, the output signal OA of amplifier 405 is tri-stated, and transistor 409 is turned on to pull the voltage of the gate terminal of transistor 201 to its source voltage, thereby turning off transistor 201. Amplifier 405 requires a relatively high operating current IAMP in order to maintain the stability of its closed-loop system. The relatively high operating current of amplifier 405 increases the current consumption of switch 101 and that of switching circuit 100 in which it may be disposed.